8.2+The+Formation+and+Nature+of+Ionic+Bonds

To bond ionically, atoms must transfer valence electrons. An atom that loses one or more electrons becomes a positive ion. An atom that gains one or more electrons becomes a negative ion. The ionic bond that forms is the electrostatic force holding the oppositely charged ions together.

The total number of electrons lost must equal the total number of electrons gained. The ratio of atoms that bond ionically must therefore be such that overall electrical neutrality is maintained.

Example: Atoms of magnesium (Mg, atomic number 12, group 2A) and chlorine (Cl, atomic number 17, group 7A) bond to form the ionic compound magnesium chloride. Use electron configurations and the balance of charges to determine the ratio of magnesium and chlorine atoms in magnesium chloride.

A neutral atom of magnesium has the electron configuration 1s2.2s2.2p6.3s2, or, in the abbreviated form, [Ne] 3s2.3p5. An atom of Mg would lose its two valence electrons to form a 2+ ion and achieve the stable electron configuration of a noble gas. An atom of Cl would gain one electron to form a 1- ion. To achieve overall neutrality in the compound, each Mg ion would require two Cl ions: (1x2+) + (2x1-) = 0. Thus, there is one Mg atom for every two Cl atoms in magnesium chloride.

Properties of ionic compounds and lattice energy: In a solid ionic compound, the positive ions are surrounded by negative ions, and the negative ions by positive ions. The resulting structure is called a crystal lattice and contains a regular, repeating, three-dimensional arrangement of ions. This arrangement, which involves strong attraction between oppositely charged ions, tends almost always to produce certain properties, such as high melting and boiling points and brittleness. Ionic compounds are always non-conductors of electricity when solid but good conductors when melted. They also act as electrolytes, substances that conduct electric current when dissolved in water. The combination of these conductivity characteristics is a very good identifier of ionic compounds, although each characteristic separately is not very reliable.

The energy required to separate one mole of the ions of an ionic compound is called lattice energy, which is expressed as a negative quantity. The greater (that is, the more negative) the lattice energy is, the stronger is the force of attraction between the ions. lattice energy tends to be grater for more-highly-charged ions and for small ions than for ions of lower charge or large size.